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.TH RANDOM 3C "Aug 14, 2002"
.SH NAME
random, srandom, initstate, setstate \- pseudorandom number functions
.SH SYNOPSIS
.LP
.nf
#include <stdlib.h>

\fBlong\fR \fBrandom\fR(\fBvoid\fR);
.fi

.LP
.nf
\fBvoid\fR \fBsrandom\fR(\fBunsigned int\fR \fIseed\fR);
.fi

.LP
.nf
\fBchar *\fR\fBinitstate\fR(\fBunsigned int\fR \fIseed\fR, \fBchar\fR \fI*state\fR, \fBsize_t\fR \fIsize\fR);
.fi

.LP
.nf
\fBchar *\fR\fBsetstate\fR(\fBconst char *\fR\fIstate\fR);
.fi

.SH DESCRIPTION
.LP
The \fBrandom()\fR function uses a nonlinear additive feedback random-number
generator employing a default state array size of 31 long integers to return
successive pseudo-random numbers in the range from 0 to 2^31 \(mi1. The period
of this random-number generator is approximately 16 x (2^31 \(mi1). The size of
the state array determines the period of the random-number generator.
Increasing the state array size increases the period.
.sp
.LP
The \fBsrandom()\fR function initializes the current state array using the
value of \fIseed\fR.
.sp
.LP
The \fBrandom()\fR and \fBsrandom()\fR functions have (almost) the same calling
sequence and initialization properties as \fBrand()\fR and \fBsrand()\fR (see
\fBrand\fR(3C)). The difference is that \fBrand\fR(3C) produces a much less
random sequence\(emin fact, the low dozen bits generated by rand go through a
cyclic pattern. All the bits generated by \fBrandom()\fR are usable.
.sp
.LP
The algorithm from \fBrand()\fR is used by \fBsrandom()\fR to generate the 31
state integers. Because of this, different \fBsrandom()\fR seeds often produce,
within an offset, the same sequence of low order bits from \fBrandom()\fR. If
low order bits are used directly, \fBrandom()\fR should be initialized with
\fBsetstate()\fR using high quality random values.
.sp
.LP
Unlike \fBsrand()\fR, \fBsrandom()\fR does not return the old seed because the
amount of state information used is much more than a single word. Two other
routines are provided to deal with restarting/changing random number
generators. With 256 bytes of state information, the period of the
random-number generator is greater than 2^69, which should be sufficient for
most purposes.
.sp
.LP
Like \fBrand\fR(3C), \fBrandom()\fR produces by default a sequence of numbers
that can be duplicated by calling \fBsrandom()\fR with 1 as the seed.
.sp
.LP
The \fBinitstate()\fR and \fBsetstate()\fR functions handle restarting and
changing random-number generators.  The \fBinitstate()\fR function allows a
state array, pointed to by the \fIstate\fR argument, to be initialized for
future use. The \fBsize\fR argument, which specifies the size in bytes of the
state array, is used by \fBinitstate()\fR to decide what type of random-number
generator to use; the larger the state array, the more random the numbers.
Values for the amount of state information are 8, 32, 64, 128, and 256 bytes.
Other values greater than 8 bytes are rounded down to the nearest one of these
values.  For values smaller than 8, \fBrandom()\fR uses a simple linear
congruential random number generator.  The \fIseed\fR argument specifies a
starting point for the random-number sequence and provides for restarting at
the same point.  The \fBinitstate()\fR function returns a pointer to the
previous state information array.
.sp
.LP
If \fBinitstate()\fR has not been called, then \fBrandom()\fR behaves as though
\fBinitstate()\fR had been called with \fIseed\fR\|=\|1 and \fIsize\fR\|=\|128.
.sp
.LP
If \fBinitstate()\fR is called with \fIsize\fR\|<\|8, then \fBrandom()\fR uses
a simple linear congruential random number generator.
.sp
.LP
Once a state has been initialized, \fBsetstate()\fR allows switching between
state arrays. The array defined by the \fIstate\fR argument is used for further
random-number generation until \fBinitstate()\fR is called or \fBsetstate()\fR
is called again. The \fBsetstate()\fR function returns a pointer to the
previous state array.
.sp
.LP
For a more powerful random number generator, see \fBarc4random\fR(3C).
.SH RETURN VALUES
.LP
The \fBrandom()\fR function returns the generated pseudo-random number.
.sp
.LP
The \fBsrandom()\fR function returns no value.
.sp
.LP
Upon successful completion, \fBinitstate()\fR and \fBsetstate()\fR return a
pointer to the previous state array.  Otherwise, a null pointer is returned.
.SH ERRORS
.LP
No errors are defined.
.SH USAGE
.LP
After initialization, a state array can be restarted at a different point in
one of two ways:
.RS +4
.TP
.ie t \(bu
.el o
The \fBinitstate()\fR function can be used, with the desired seed, state array,
and size of the array.
.RE
.RS +4
.TP
.ie t \(bu
.el o
The \fBsetstate()\fR function, with the desired state, can be used, followed by
\fBsrandom()\fR with the desired seed. The advantage of using both of these
functions is that the size of the state array does not have to be saved once it
is initialized.
.RE
.SH EXAMPLES
.LP
\fBExample 1 \fRInitialize an array.
.sp
.LP
The following example demonstrates the use of \fBinitstate()\fR to intialize an
array. It also demonstrates how to initialize an array and pass it to
\fBsetstate()\fR.

.sp
.in +2
.nf
# include <stdlib.h>
static unsigned int state0[32];
static unsigned int state1[32] = {
     3,
     0x9a319039, 0x32d9c024, 0x9b663182, 0x5da1f342,
     0x7449e56b, 0xbeb1dbb0, 0xab5c5918, 0x946554fd,
     0x8c2e680f, 0xeb3d799f, 0xb11ee0b7, 0x2d436b86,
     0xda672e2a, 0x1588ca88, 0xe369735d, 0x904f35f7,
     0xd7158fd6, 0x6fa6f051, 0x616e6b96, 0xac94efdc,
     0xde3b81e0, 0xdf0a6fb5, 0xf103bc02, 0x48f340fb,
     0x36413f93, 0xc622c298, 0xf5a42ab8, 0x8a88d77b,
     0xf5ad9d0e, 0x8999220b, 0x27fb47b9
     };
main() {
     unsigned seed;
     int n;
     seed = 1;
     n = 128;
     (void)initstate(seed, (char *)state0, n);
     printf("random() = %d0\en", random());
     (void)setstate((char *)state1);
     printf("random() = %d0\en", random());
}
.fi
.in -2

.SH ATTRIBUTES
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	See \fBNOTES\fR below.
.TE

.SH SEE ALSO
.LP
.BR arc4random (3C),
.BR drand48 (3C),
.BR rand (3C),
.BR attributes (7),
.BR standards (7)
.SH NOTES
.LP
The \fBrandom()\fR and \fBsrandom()\fR functions are unsafe in multithreaded
applications.
.sp
.LP
Use of these functions in multithreaded applications is unsupported.
.sp
.LP
For \fBinitstate()\fR and \fBsetstate()\fR, the \fIstate\fR argument must be
aligned on an \fBint\fR boundary.
.sp
.LP
\fBarc4random\fR(3C) is a newer and better performing random number generator.
Use it instead.
